the SKS pool documentation points out an onionbalance load balancer that can distribute requests over all the known tor onion services.
There are actually more keyservers that offer onion services than there are that offer hkps, afaict.
If dirmngr's default were to include hkp://jirk5u4osbsr34t5.onion as well as hkps://hkps.pool.sks-keyservers.net then the tor auto-detection (or an explicit use-tor) could make use of the onionbalance. This would spread the load more evenly among the existing keyservers.